Eocelia: morphological evolution

Morphological evolution in a lineage of brachiopods, presented as an illustration of the ambiguity in interpreting whether the process is gradual or punctuational. From the statistical analysis of fossil shells detailed in steps A through D, one may conclude that periods of essentially no change in shell rib strength, each lasting millions of years, are interspersed with comparatively short bursts of rapid change. From another point of view, however, one may see the same record as evidence of an unbroken process of evolution in which the rate of change speeds up somewhat at particular times.